Heretofore, JP10-203338A, which corresponds to U.S. Pat. No. 006,113,197A, proposes a vehicle brake control apparatus of a brake-by-wire type, which has four pumps respectively for four wheels of a vehicle. Two of the four pumps are located in a first conduit system and are driven by a common motor, and the other two are located in a second conduit system and are driven by the other common motor.
In the above-described brake-by-wire type vehicle brake control apparatus, W/C pressure of each vehicle wheel is generated on the basis of a discharge amount of brake fluid of each pump driven by the corresponding motor. In such structure, the W/C pressure of each vehicle wheel would not rise (increase) until the amount of brake fluid discharged by the pump reaches a certain level. The brake fluid amount, which is insufficient to increase the W/C pressure, is herein referred to an ineffective fluid amount. Such ineffective fluid amount may impair the braking response at an initial brake operation.
In order to improve such braking response at the initial stage, it is preferable to always drive the motor to the full extent, i.e., at a rotation speed appropriate to discharge the brake fluid.
Meanwhile, since such brake-by-wire type brake control apparatus requires two motors driven with a larger electric power supplied from a battery, it is desirable to reduce the power consumption as much as possible. Therefore, it is not preferable to consider only the braking response at the initial stage and to always drive the motors to the full extent.
In the light of the foregoing, it is necessary to reduce motor power consumption while improving the braking response at the initial brake operation.
As one possible method of coping with the above, a motor can be driven to the full extent until a discharge amount of brake fluid from the pump reaches a certain level exceeding an ineffective fluid amount. After the discharge amount reaches the certain level, the amount of the electric power supplied to the motor can be reduced in response to a target braking force.
However, when the discharge amount of brake fluid from the pump reaches the certain level exceeding the ineffective fluid amount, pressure differential between a target W/C pressure and an actual W/C pressure has already been created. Accordingly, the braking response at the initial brake operation may not be properly improved.
The present invention has been made in view of the above description and provides a brake-by-wire type brake control apparatus for a vehicle, in which braking response at an initial brake operation is improved and the power consumption by a motor is reduced.